This invention relates generally to the direct reduction of iron oxide material and, more specifically, the present invention relates to the removal of carbon dioxide from spent reducing gas in a process for the direct reduction of iron.
Direct reduced iron (DRI) production represents one of the major routes to producing steel. The actual reduction of the iron ore in the direct reduction reactor is carried out in the presence of a reducing gas that comprises hydrogen and carbon monoxide. In the DRI process, the iron is reduced and the carbon dioxide, produced in the reduction reaction, is removed with the spent reducing gas, or reactor off-gas. The reactor off-gas includes unreacted hydrogen, carbon monoxide, carbon dioxide and water. After cooling, the reactor off-gas is vented or reprocessed to remove the carbon dioxide and enrich the hydrogen and carbon monoxide content before returning the enriched off- gas to the direct reduction reactor as the reducing gas. Some schemes use a reforming step to provide more hydrogen and carbon monoxide and some schemes use a water gas shift step to enhance the recycle gas to provide the reducing gas. All of the schemes must remove carbon dioxide to maintain the reduction process.
The direct reduction of iron ore, i.e. iron oxides, is accomplished by reduction of the iron ore by reaction with carbon monoxide, hydrogen and/or solid carbon through successive oxidation states to metallic iron. Typically, oxides of iron and carbonaceous material, e.g. coal, are charged into a furnace. Heat is supplied to the furnace by the combustion of fuel with air to generate, inter alia, carbon monoxide. As the iron ore and reducing agents pass through the furnace, the iron ore is reduced to metallic iron and recovered from the furnace. Furnace gases are removed from the furnace through a flue or exhaust conduit.
Direct reduction plants for producing direct reduced iron, known as DRI (sponge iron) or hot briquetted iron (pre-reduced materials useful as feedstocks for iron and steel making), currently produce such products by contacting a reducing gas, composed principally of hydrogen and carbon monoxide, at effective reduction temperatures in the range from about 750xc2x0 to about 1050xc2x0 C., over a bed of particulate iron-containing material in the form of lumps or pellets. Examples of such processes are described in U.S. Pat. No. 3,749,386, U.S Pat. No. 3,764,123, U.S Pat. No. 3,816,101, U.S Pat. No. 4,336,063, U.S Pat. No. 4,556,417, U.S Pat. No. 5,078,787, U.S Pat. No. 4,046,557, U.S Pat. No. 4,002,422 and U.S. Pat. No. 4,375,983.
U.S. Pat. No. 5,238,487 discloses a process for producing direct reduced iron in a reduction reactor fed with top gas effluent from a first reaction reactor attached to a melter-gasifier. The invention of the ""487 patent addresses the problems related to carburization and metal dusting of the heater pipes which arise when heating a reducing gas containing a high content of carbon monoxide in a heat exchanger type direct fired heater. More particularly, the ""487 patent teaches the withdrawal of oxidants in the top gas effluent and the use of the top gas effluent in a second reduction reactor. A reducing gas is withdrawn from the second reduction reactor. The reducing gas is passed in turn to a cooler/scrubber, a CO2 removal unit, a gas heater to heat the gas to a temperature between 200xc2x0 and 500xc2x0 C., and a partial combustion chamber to heat the gas to a temperature between 750xc2x0 and 850xc2x0 C., before being returned to the second reactor.
U.S Pat. No. 5,882,579 discloses a method and an apparatus for utilizing in a second reduction reactor the excess exhausted gas from a first reduction reactor fed with a reducing gas produced in a melter-gasifier without the problems of swelling presented by iron ores when reduced by carbon monoxide, thus increasing the capabilities of the reduction system to process a wider range of iron ores. It is disclosed therein that the method for producing DRI comprises providing a source of reducing gas having a high content of carbon monoxide, of about 30% to about 40%. The reducing gas flows through a first reduction reactor where iron ore is pre-reduced and the pre-reduced iron ore is fed to the source of reducing gas for melting and removal as pig iron. Top gas effluent from the first reduction reactor is cooled and cleaned by adding water to the relatively cool and clean gas stream and feeding it to a catalyst-laden vessel to carry out the reaction of carbon monoxide with the water to produce hydrogen and carbon dioxide. Carbon dioxide is removed from the reducing gas, thus producing a reducing gas stream with a hydrogen content above about 65% and utilizing the reducing gas in a second reduction reactor.
U.S Pat. No. 5,858,057 which is hereby incorporated by reference, discloses a similar process for the production of direct reduced iron, wherein the amount of carbon in the direct reduced iron process is controlled by modifying the relative amounts of water, carbon dioxide and oxygen in the composition of the reducing gas returned to the reduction reactor. It is disclosed that the amount of carbon in the direct reduced iron reactor is controlled by the amount of water in the reducing gas, and that the addition of oxygen to the reducing gas provides the energy for the carburization of the direct reduced iron product.
U.S. Pat. No. 3,909,244 discloses a process for modifying the composition of a reducing gas produced from the steam reforming of natural gas reformer to enrich the reducing gas with hydrogen. The rate of reduction is increased by reducing iron ores with a gas essentially composed of hydrogen, and the economy of the regenerating of the recirculating gases is improved.
U.S. Pat. No. 4,363,654 relates to a process for producing a reducing gas for a direct reduction or a blast furnace wherein oil and/or coal is partially oxidized in the presence of air to produce a reducing gas stream containing hydrogen and nitrogen together with other gases. The reducing gas stream is treated to remove essentially all gases other than hydrogen, and the hydrogen is used in reducing iron ores.
Because the reactor off-gas from the DRI reactor is generally produced at low pressure and high temperature, separation methods such as conventional pressure swing adsorption (PSA), or liquid absorption, require that the reactor off-gas be cooled and that the pressure of the cooled reactor off-gas be raised significantly to obtain the necessary driving force to carry out the separation. Liquid absorption systems require steam generation and water treatment systems to comply with environmental regulations. In addition, such solvent-based systems have associated solvent handling and disposal problems. If the pressure of the separation is not raised and an adsorbent-based separation is used, the desorption of the adsorbent must be carried out at a desorption pressure below atmospheric pressure, subjecting a stream comprising hydrogen and carbon monoxide to potentially explosive conditions. Such alternatives are both expensive and potentially explosive. Processes are sought which provide carbon dioxide removal from DRI reactor off-gas at low operating pressure without the danger of explosion.
Applicant discovered that a PSA process which uses an external purge step following a depressurization step provides an effective way of carrying out the separation of carbon dioxide from DRI spent reducing gas at very low adsorption pressures without requiring a substantial lower desorption pressure. The present invention solves the above problems by using natural gas to purge and desorb CO2 from an adsorption zone. Natural gas, instead of reducing gas, is first used to purge most of the CO2 and water from the adsorption zone. A small amount of product purge may be used after the natural gas purge step to remove some methane and additional CO2 from the adsorption zone. The natural gas used to provide the external purge gas is not lost. A portion of the natural gas is recovered in the reducing gas and recycled to the reactor, while the remaining portion is combined with a CO2-rich stream, which can be used as fuel gas. Preferably, the external purge step is combined with an internal purge step wherein a portion of the product stream is used to purge the adsorbent bed either prior to, or following the external purge step. By use of the external purge step at the lowest pressure in the pressure swing cycle, the adsorbent is surprisingly stripped of adsorbed species to a point resulting in increased carbon dioxide removal rates relative to conventional pressure swing or vacuum swing adsorption units without the need for vacuum equipment or large blowers. The external purge step of the present invention serves to more completely sweep the adsorbent beds of carbon dioxide to a greater extent than conventional PSA processes. This unexpectedly provides enhanced carbon dioxide removal and an improved effective working capacity of the adsorbent relative to the same adsorbent in a conventional PSA process.
In one embodiment, the present invention is a process for the production of direct reduced iron. The process comprises contacting an iron ore stream with a reducing gas stream at effective reducing conditions in a reaction zone and recovering a direct reduced iron product and a reactor off-gas stream. The reducing stream comprises hydrogen and carbon monoxide. The reactor off-gas stream is cooled to provide a cooled reactor off-gas stream. At least a portion of the cooled reactor off-gas stream is passed as a feed stream to a PSA zone. The PSA zone comprises at least two adsorbent beds, wherein each adsorbent bed undergoes a cyclic process comprising an adsorption step, a co-current depressurization step, a counter-current depressurization step, an external purge step and a repressurization step to provide a reducing gas product stream and a tail gas stream. In the cyclic PSA process, the external purge step is conducted following the depressurization step. The reducing gas product stream comprises hydrogen and carbon monoxide and is produced during the adsorption step. The tail gas stream is produced during the depressurization and external purge steps. The reducing gas product stream is heated in a furnace to provide the heated reducing gas stream, which is then sent to the DRI reactor as reducing gas stream.
In another embodiment, the present invention is a process for the low pressure production of direct reduced iron. The process comprises passing a reducing stream and iron ore to a direct reduction reaction zone at effective reduction conditions to produce a direct reduced iron product and a reactor off-gas stream. The reactor off-gas stream is quenched and cooled to provide a cooled off-gas stream comprising hydrogen, carbon monoxide, carbon dioxide and water. At least a portion of the cooled off-gas stream is passed at effective adsorption conditions as a feed stream to a pressure swing adsorption zone comprising at least two adsorbent beds. Each adsorbent bed undergoes a cyclic process consisting of an adsorption step, a co-current depressurization step, a counter-current depressurization step, an external purge step and a repressurization step. In the cyclic process, the external purge step is conducted following the depressurization step to provide a reducing gas product stream comprising hydrogen and carbon monoxide in the adsorption step and to provide a tail gas stream in the depressurization and external purge steps. A water stream is admixed with a remaining portion of the cooled off-gas stream and a hydrocarbon stream to provide a reforming zone feed stream. The reforming zone feed stream is passed to a reforming zone which is indirectly heated by the combustion of a fuel admixture comprising at least a portion of the tail gas stream, an oxygen containing stream and a fuel stream to produce a reformer effluent stream. The reformer effluent stream and the reducing gas product stream are admixed to provide the reducing stream.